fixes for plasmawindowed sni support

Summary:

• support one sni per applet
• fix commandline parsing when the process is already running (--statusnotifier paramenter was breaking parsing)
• sync applet and sni status
• toggle shown/hidden based on current window status
• close action closes the plasmoid, not the whole app
• applet contextual action in context menu

Test Plan:
tested with different applets, more than once,
context menus correctly synced, status correctly synced
can quit plasmoids one by one

Reviewers: apol, davidedmundson

Reviewed By: davidedmundson

Subscribers: apol, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D5242